Reinforced filter tube and method of making the same

ABSTRACT

There is disclosed a porous, vacuum formed filter tube having randomly oriented glass fibers and having at least one layer of a suitable sheet material wrapped around said porous filter tube and being in intimate contact therewith, said disclosed combination having an outer support structure of a predetermined inside diameter sufficient to compress the assembly of said porous filter tube and said material when it is slipped thereover, thus providing compression from the outside to the inside and forcing the layer of sheet material into intimate contact at virtually all points with said porous filter tube.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to an improved reinforced coalescingfilter tube which may be used in virtually any coalescing filterassembly, and more particularly relates to a coalescing filter tubehaving a layer of nonwoven material interposed between a vacuum formedfilter layer and a reinforcing structure to prevent expansion of thevacuum formed filter layer into the openings in the support structureduring use, a situation which has been found to be undersireable becausethe packing density and, therefore, the pore size of the filter at theopenings in conventional constructions has been found to be larger thanthe packing density and pore size elsewhere in the filter. In addition,the providing of the layer of nonwoven material gives added support inhigh differential pressure situations.

2. Description of the Prior Art

Most tubular filters of the type with which the present invention isconcerned are made for use in filter housings, and are used to floweither in-to-out or, from the outside in. While it is advantageous toflow from the outside in for many filter applications, there is also adefinite advantage for flowing in-to-out for certain applications. Forexample, the coalescing of liquid droplets and aerosols from gases, orthe coalescing of two liquid phases. In these applications, it isdesireable to have an external support structure to support the filtermedia and thereby prevent media rupture caused by high differentialpressure across the filter. Such external support structures are usuallymade of metal or plastic.

Filter tubes of this type are commonly manufactured by applying a vacuumto the inside of a porous mandrel and submerging the mandrel in a slurryof fibers of various compositions. The composition of the slurrydetermines the pore size of the filter. Because of the vacuum applied tothe mandrel, the fibers in the slurry are deposited on the surface ofthe mandrel, the mandrel is then removed from the slurry, and after anyfree or excess water is removed, a filter tube is left on the mandrel.

The inside diameter of the filter tube is very consistant from filtertube to filter tube because the inside diameter of the tube is afunction of the outside diameter of the mandrel. It, additionally, isvery smooth and uniform in appearance because it is formed against themandrel.

However, the outside diameter not only is not uniform, but is very roughin appearance because it does not have any similar structure to formagainst. This has produced a serious problem in the art of how to applya support structure to the outside of the tube, and have it in contactwith said tube at all points, so that the filter tube does not rupturewhen pressure is applied thereto.

In order to prevent the filter tube from bursting, it is essential thatthe filter media is supported by an external support structure inrelatively intimate contact with the media. The optimum situation wouldbe to have a support structure in contact with the filter media at aninfinite number of points around the outside diameter of the filtertube. However, previous attempts in providing outer support structureshave not been entirely satisfactory.

Basically, four methods have been tried. The first method involvedplacing an outer support structure loosely over the filter media.However, this method does not provide close intimate contact between thefilter media and the support core. Thus, rupture of the filter media wasvery likely to occur at even low differential pressures across thefilter media in the neighborhood of 10 to 25 PSID.

The second method involved compressing the filter media between an innerrigid support core and an outer rigid support. This method requires bothan internal and an external support structure to be utilized, with saidouter support structure having a clapping means for maintaining itsposition relative to the inner support structure, and for maintainingcompression of the filter media. An example of this method can be seenin the U.S. Pat. No. 3,460,680 to Domnick. This method usually requiredstainless steel support structures for prevention of corrosion, andwhile it is used to the present day, it is very costly to manufacture,and still does not permit total contact between the filter media and thesupport structure. For this reason, it is still not satisfactory formany applications.

A third method, involving placing a rigid support structure over thetube without any outward force applied from the inside of the filtertube was tried. However, this method suffers from two deficiencies.There is (1) a lack of intimate contact between the filter media and thesupport structure, or (2) if the support structure is too small inrelation to the outer diameter of the filter tube, there is damage tothe filter media while attempting to slip on a support structure. Thus,this method still leaves a serious problem in the prior art.

The latest attempt at solving these problems in the art involves afourth method where the filter media is brought in intimate contact witha rigid outer support core as a result of an outwardly directed forcehaving been supplied to the internal surface of the filter media duringthe manufacturing process. One embodiment of this method can be seen inthe U.S. Pat. No. 4,052,316 to Berger et. al. This method utilizes acontinuous rigid support structure, such as a plastic or metalperforated core, which is slipped over the filter media while the mediais still under vacuum on a mandrel. After the tube has been slipped overthe media, the vacuum is released, and an outward pressure of air forcesthe media into intimate interlocking contact with the outer supportstructure. It actually forces the media into the openings of the supportcore for an interlocking contact.

The pore size and structure of a filter media is a function of therelative surface area of that filter media which, in turn, is a functionof the median fiber diameter and packing density. In those areas wherethe filter media has been forced into the openings of the outer supportstructure, the thickness of the media will be greater than the adjacentmedia which is in contact with the support structure (see FIG. 2).Hence, the packing density of the filter media in the opening will beless than the packing density of the filter media in contact with thesupport structure, and this creates a lack of uniformity in the filtermedia with regard to pore size and structure.

Additionally, there is a lack of support of the filter media in theopenings of the outer support structure. Therefore, when higherdifferential pressures are applied from the inside to the outside of thefilter media, distortion, or even rupture, of the filter media willoccur sooner than in supported areas.

SUMMARY OF THE INVENTION

To solve the problems in the prior art, a porous vacuum formed filtertube is provided having randomly oriented glass fibers with at least onelayer of a suitable sheet material wrapped around said porous filtertube and being in intimate contact therewith, said combination having anouter support structure of a predetermined inside diameter sufficient tocompress the assembly of said porous filter tube and said sheet materialwhen it is slipped thereover, thus providing compression from theoutside to the inside, and forcing the layer of sheet material intointimate contact at virtually all points with said porous filter tube.

Thus, it is one of the objects of the present invention to provide atubular filter having an external support structure in relativelyintimate contact with the media at an infinite number of points aroundthe outside diameter of the tube.

A further object of the present invention is to provide a tubular filterhaving an external support structure, and having a uniform wallthickness throughout the entire filter tube.

A still further object of the present invention is to provide a tubularfilter having an external support structure, and having a nonwoven sheetmaterial interposed between said tubular filter and said externalsupport structure to provide a filter tube having a relatively uniformwall thickness, and being in intimate contact with virtually all pointsof said nonwoven material which, in turn, is supported by a rigid outersupport structure.

A still further object of the present invention is to provide a filterof the foregoing nature wherein said sheet material is of a spun bondedstructure having high tensile and tear strength, excellent dimensionalstability, no media migration, and good chemical resistance.

A still further object of the present invention is to provide a filtertube having both an external support structure and an internal supportstructure.

A still further object of the present invention is to provide a filtertube of the foregoing nature, whether or not also supported internally,and having an external drain layer.

Another object of the present invention is to provide a filter tube ofthe foregoing nature which may be easily installed in commerciallyavailable filter assemblies.

Further objects and advantages of this invention will be apparent fromthe following description and appended claims, reference being had tothe accompanying drawings forming a part of the specification, whereinlike reference characters designate corresponding parts in the severalviews.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an end view of a prior art filter tube, showing the intimateinterlocking contact between the outer support structure and the filtertube.

FIG. 2 is an enlarged sectional view of the prior art shown in FIG. 1,showing how the packing density of the filter media varies in theportions of the filter tube which have expanded out into the openings ofthe outer support structure of the prior art tube.

FIG. 3 is a perspective view, partially broken away, showing aconstruction embodying our invention.

FIG. 4 is an enlarged view, partly in section, of the construction shownin FIG. 3 showing how the addition of the wrapped layer between thefilter tube and the outer support core of a construction embodying ourinvention maintains uniform packing density of the filter tube.

FIG. 5 is a block diagram showing the series of steps necessary to formthe construction shown in FIG. 3.

FIG. 6 is an end view of a modification of the construction shown inFIG. 4 where, in addition to the outer support structure, there isprovided an inner support structure having an inner layer of sheetmaterial between it and the filter tube.

FIG. 7 is a block diagram showing the additional steps necessary, overthose shown in FIG. 5, to form the construction shown in FIG. 6.

It is to be understood that the invention is not limited in itsapplication to the details of construction and arrangement of partsillustrated in the accompanying drawings, since the invention is capableof other embodiments, and of being practiced and carried out in variousways within the scope of the claims. Also, it is to be understood thatthe phraseology and terminology employed herein is for the purpose ofdescription, and not of limitation.

Referring to FIGS. 1-4, the available prior art filters, and the filterof the present invention can be viewed for comparison purposes. FIGS. 1and 2 show the prior art devices.

As can be seen in FIG. 2, the placing of an outer support structurearound a vacuum formed filter tube, and then releasing pressure and/orapplying a positive outward pressure does force the filter media intothe openings of the outer support structure for a distance of Y. Wherethis happens, the packing density and, thus, the pore size of thefilters becomes much greater, and is not uniform, as compared to thedensity of the filter tube where it is supported at all points by thesupport structure.

In contrast, the filter of the present invention can be seen byreferring to FIGS. 3-4. If a filter tube with an outer support structureonly is desired, this is shown in FIG. 3, wherein there is shown afilter tube 20, surrounded by a layer of nonwoven media 21, which issurrounded by an outer support structure 22. It can be seen that theinner wall 20A of the filter tube is uniform because of the fact that itis supported on a mandrel while forming. The outer wall 20B is also keptrelatively uniform because it is wrapped with the nonwoven media 21while still wet. The outer support structure 22 is slipped over thecombination of the filter tube 20, and the nonwoven media 21, after thevacuum forming process takes place, as will be explained hereinafter.

FIG. 6 shows the construction of the present invention when an innersupport structure is also desired. In this case, an inner supportstructure 25 is first provided which is wrapped with an inner layer ofnonwoven media 26, after which the filter tube 20, the nonwoven media21, and the support structure 22, previously described, are provided.

Of course, it should be understood that it is well within the scope ofthe present invention to provide the inner support structure 25, and theinner layer of non-woven media 26, having a filter tube 20 formedthereon, without providing the outer layer of nonwoven media 21, and theouter support structure 22, if desired.

The method of manufacture of our improved filter tube can be seen byreferring to FIGS. 5 and 7.

Whether manufacturing the filter of the present invention with only theouter support structure as shown in FIG. 3, or with the inner supportstructure 25 and the outer support structure 22 as shown in FIG. 6, thefirst step is to provide a slurry (Box 200 or Box 310). The manner ofpreparing the liquid slurry of fibers is well known in the art and neednot be described here in detail. The slurry will consist of water andglass fibers and may contain an emulsion binder to give the formedfilter additional strength. The slurry will have a proper composition ofglass fibers to determine the proper pore size, and will have the properamount of glass fibers therein for optimum forming time. The amount, byweight, of fibers does not determine the pore size of the filter oreffect the same unless outside the generally accepted percentages byweight of fibers to the portion of water used in the slurry. The time ofmixing of the slurry does determine the length of the fibers and thesmoothness of the outer wall, and should be chosen for optimumproduction efficiency.

The next step is to provide a mandrel (Box 210 or Box 320) and thenconnect a suitable vacuum pump to provide vacuum to the mandrel (Box 230or Box 330). The providing of the mandrel, and the connecting of avacuum pump to the mandrel are too well known in the art to requirediscussion herein, except for a few general comments which are importantto the particular process being described.

The mandrel may be the same as those commonly used to manufacturetubular vacuum formed filters. The particular size and shape of themandrel will depend upon the particular size filter tube beingmanufactured. If the inner support structure 25, is desired (Box 220)this is first wrapped (Box 230) with one or two layers of non-wovenmedia, 21. The free end of the media may be attached to the body of themedia, if desired, either by a heat sealing process, or by a hot meltadhesive. If a hot melt adhesive is used, it is preferable that if be ofthe same type as the nonwoven material. For example, if you are using anonwoven spun bond polyester material, one would choose a polyester hotmelt. While this is preferable, it is not absolutely necessary becausein some applications it may be desireable to use differing materials.For example, you could use a polypropylene hot melt with a polyesterspun bonded material.

Whether you are using a heat seal technique or a hot melt adhesivetechnique, the temperature resistance of the means of attachment iscritical. If you have a filter tube which will be used at 300 degreesFahrenheit, you obviously could not use a polypropylene hot meltadhesive which melts at approximately 220 degrees.

It should be understood that it is well within the scope of theinvention that the internal support structures 25 could be prewrappedwith the nonwoven material to speed the production process and bealready prepared for the operator to place on the mandrel (Box 240). Itis not necessary to wrap the support structure while it is on themandrel.

At this stage, whether the inner support structure is being used, orwhether we are manufacturing a filter tube with an outer supportstructure only, a vacuum will be provided to the mandrel by methods wellknown in the art, which do not need additional description herein.

The submerged mandrel (Box 260 or 340) will be left in the slurry mixfor a preset period of time, or until a preset amount of pressurerestriction in the vacuum line appears.

The mandrel is then removed from the glass fiber slurry mix and thevacuum is maintained for a preset period of time, (Box 280, 370)depending on the size of the filter tube, to remove free and excesswater. After this, the vacuum will be turned off (Box 290, 380) and theassembly so formed will be removed (Box 300, 390) from the mandrel. Ifthe filter assembly is one which had the inner support core 25, it maybe handled as it comes off of the mandrel for further operations. If theinner support 25, and the inner layer of nonwoven media 26 are absent,it is preferable to place the raw filter tube on a teflon coated supportmandrel while wrapping (Box 410) the tube with the outer layer of thenonwoven media 21. Both the inner layer of nonwoven media 26, and theouter layer of nonwoven media 21, may be such as a spun bondedpolyester, polypropylene, or polyolefin, although any nonwoven materialmay be used.

The use of a nonwoven material is preferred because of the sheetstructure of continuous filament fibers randomly arranged, highlydispersed, and bonded at the filament junctions. The spun bondedstructure further provides a high tensile and tear strength, excellentstability, no media migration, good chemical resistance andpermeability. It is contemplated however, that in some instances a wovenmaterial may be suitable, and the use of a woven material is well withinthe scope of the present invention.

It is important to the process of the present invention that the presettime or preset amount of pressure restriction in the vacuum line bechosen such that the wall thickness of the particular filter beingformed, whether it has one or two layers of spun bonded material appliedthereon, will be slightly larger than the inside diameter of the outersupport structure 22. By "slightly larger" is meant that the filter tubeis formed slightly larger than the inside diameter of the retainer byseveral thousandths of an inch. This, in combination with the nonwovenfiber, which adds several thousandths more of an inch, together arelarger than the inside diameter of the retainer. Together they should belarger than the inside diameter of the retainer by approximately tenthousandths to eighty thousandths of an inch, depending upon theapplication and amount of compression of the media desired.

The outer support structure is seamless, and must be of sufficientrigidity to maintain the radially inward compression on the combinationof the nonwoven fiber and the filter tube to properly perform itssupport function.

It is also important to select the material (Box 440) of which the outersupport structure 22, is constructed such that it has a low coefficientof friction with the spun bonded material, such as a polypropylene orpolyethylene material. In this way, the outer support structure 22, maybe slipped over (Box 450) the combination of the inner support structure25, inner layer of nonwoven media 26, filter tube 20, and nonwoven media21, if a filter tube with an inner support structure is used, or overthe filter tube 20, and the outer layer of nonwoven material 21, if afilter tube with an outer support only is being formed. Since the outerdiameter of the combination of the tube and media is slightly largerthan the inner diameter of the outer support structure 22, when theouter support structure 22 is slipped over the combination one will becompressing this assembly from the outside in a radially inwarddirection to achieve intimate contact between the outer supportstructure 22, and the outer layer of nonwoven media 21. Because of thestrength of the nonwoven media 21, the filter tube 20 will not be ableto expand out into the openings in the outer support structure.

Of course, if the inner support 25, and the inner layer of nonwovenmedia 26 are used, these are also placed in compression so that thefilter tube 20 is in intimate contact with the inner layer 26.

To complete the manufacture of the filter assembly it is now taken offthe mandrel and may be impregnated with a resin binder (Box 470) bymeans well known in the art, and is then cured (Box 480) to harden theresin and thus make the filter tube 20, suitable for everyday use.

If desired, a foam or glass drain layer may be applied about the outsideof the outer support structure 22 (Box 490), whether or not thepreviously manufactured assembly has been impregnated with the resinbinder and cured.

Thus, by carefully analyzing the problems present in the prior art, andeliminating them by the addition of a layer of nonwoven spun bondedmaterial interposed between an inner or outer support structure and afilter tube, we have provided an improved filter tube and method ofmanufacturing the same.

We claim:
 1. A filter including, in combination:(a) a porous filter tube formed of randomly oriented glass fibers, (b) at least one layer of a suitable thin sheet material wrapped around said porous filter tube and being in intimate contact therewith, and (c) a seamless outer tubular support structure of sufficient rigidity to radially inwardly compress said porous filter tube and said suitable thin sheet material and having an inside diameter slightly smaller than the outside diameter of the combination of the porous filter tube and said outer layer of sheet material and completely surrounding said combination, said support structure having a low coefficient of friction in relation to said suitable thin sheet material thereby holding said combination in compression, and causing said outer layer of sheet material to be in intimate contact at virtually all points of said porous filter.
 2. A filter, including in combination;(a) a seamless inner tubular support structure, (b) an inner layer suitable sheet material wrapped around said tubular support structure, (c) a porous filter tube formed of randomly oriented glass fibers, (d) at least one outer layer of a suitable thin sheet material wrapped around said porous filter tube, and being in intimate contact therewith; and (e) a seamless outer tubular support structure of sufficient rigidity to radially inwardly compress said porous filter tube and said suitable thin sheet material and having an inside diameter slightly smaller than the outside diameter of the combination of said inner tubular support structure, said inner layer of suitable sheet material, said porous filter tube, and said outer layer of suitable sheet material and completely surrounding said combination, said support structure having a low coefficient of friction in relation to said suitable thin sheet material, thereby holding said combination in compression, and causing said outer layer of sheet material to be in intimate contact at virtually all points with said porous filter tube.
 3. The filter defined in claim 1, wherein the free end of said outer layer of suitable sheet material is reattached to said layer by a hot melt adhesive.
 4. The filter defined in claim 3, wherein said hot melt adhesive is a polyester hot melt.
 5. The filter defined in claim 1, wherein said outer support structure is a perforated retaining tube.
 6. The filter defined in claim 5, wherein said perforated retaining tube is plastic.
 7. The filter defined in claim 5, wherein said perforated retaining tube is metal.
 8. The filter defined in claim 5, and further including a drain layer surrounding said outer support structure.
 9. The filter defined in claim 8, wherein said drain layer is a porous foam.
 10. The filter defined in claim 8, wherein said drain layer is fiber felt.
 11. The filter defined in claim 2, wherein said inner layer of suitable sheet material and said outer layer of suitable sheet material both have their free ends reattached by a hot melt adhesive.
 12. The filter defined in claim 2, wherein said inner layer of suitable sheet material and said outer layer of suitable sheet material both have their free ends reattached by a heat sealing process.
 13. The filter defined in claim 11, wherein said hot melt adhesive is a polyester hot melt.
 14. The filter defined in claim 2, wherein said outer support structure is a tubular perforated retaining tube.
 15. The filter defined in claim 14, wherein said perforated retaining tube is plastic.
 16. The filter defined in claim 14, wherein said perforated retaining tube is metal.
 17. The filter defined in claim 14, and further including a drain layer surrounding said outer support structure.
 18. The filter defined in claim 17, wherein said drain layer is porous foam.
 19. The filter defined in claim 17, wherein said drain layer is fiber felt.
 20. A filter assembly comprising a filter as described in either one of claims 1 or 2, and including means for introducing the fluid to be filtered to the interior of said filter at the inside of the innermost filter layer in such a manner that such fluid will flow through remaining layers of said filter, wherein said filter has its ends substantially closed by a pair of closure members, and is mounted inside a suitable filter housing having inlet means sealingly communicating with the interior of said filter, and outlet means communicating with the atmosphere. 